1. Field of the Invention
The present invention relates to optical fiber communication systems, and in particular relates to an optical fiber communication system having a reflective optical network unit.
2. Description of the Related Art
FIG. 1A is a diagram of an optical fiber communication system. As shown in FIG. 1A, the optical fiber communication system 100 is a wavelength-division multiplexing-passive optical network (WDM-PON) system. The plurality of the laser sources L1, L2 . . . Ln in the central office (CO) 110 (or called head-end) respectively generate light sources having different wavelengths λ1, λ2, . . . λn. The light sources are integrated into an optical carrier DS by the optical multiplexer 111 of the array wave guide (AWG). The multiplexer 111 is coupled to an optical circulator 112. The first terminal of the optical circulator 112 receives the optical carrier DS and delivers the optical carrier DS to the fiber 113 coupled to the second terminal of the optical circulator 112. The third terminal of the optical circulator 112 is coupled to the optical demultiplexer 114 of the central office 110. The optical demultiplexer 114 delivers the upstream signal US to a corresponding receiver 115 by the fiber 113 and the optical circulator 112.
In addition, the optical carrier DS is separated by an optical demultiplexer 121 into user devices (e.g., user device 122). A reflective optical network unit (RONU) 120 having at least one reflective modulator 123 is established, in which the reflective modulator 123 reuses the optical carrier DS and delivers the upstream signal US of the client to the receiver 115 of the central office 110.
However, the cross-sectional area of the fiber becomes an ellipse shape due to manufacturing process or procedures for establishing an optical fiber communication system. Thus an optical fiber communication system with loop back network structures may more easily generate interference noise from Rayleigh backscatter (RB) effect, which especially affects the transmission of the upstream data. In a nutshell, the portions of the light signal or the radio frequency signal delivered in fibers are constantly reflected by the fibers. Finally, the reflected signals delivered to the receiver 115 of the central office 110 become Rayleigh backscatter noise.
FIG. 1B is another diagram of the optical fiber communication system of FIG. 1A. As shown in FIG. 1B, carrier Rayleigh backscattering CRB and signal Rayleigh backscattering SRB are two main types of Rayleigh backscatter noises. Carrier Rayleigh backscattering CRB is generated by the optical carrier DS, and signal Rayleigh backscattering SRB is generated by the upstream signal US. The carrier Rayleigh backscattering CRB is generated by the process where the optical carrier DS is delivered from the central office 110 to the optical demultiplexer 121. The signal Rayleigh backscattering SRB is generated by the process where the Rayleigh backscattering RB, generated when the optical carrier DS is delivered from the central office 110 to the optical demultiplexer 121, is modulated again by the reflective modulator 123 of the reflective optical network unit 120 and then delivered to the receiver 115 of the central office 110.
Therefore, there is a need for an optical fiber communication system to decrease the carrier Rayleigh backscattering CRB and the signal Rayleigh backscattering SRB for improving a signal transmission.